The present invention relates to a two-stroke internal combustion engine which is suited for use in a portable power working machine, and in particular to a two-stroke internal combustion engine which is capable of reducing the number of pieces constituting the main body which is essentially consisted of a cylinder portion and a crankcase portion, thereby making it possible to reduce the number of parts such as assembling bolts, etc. and hence to reduce the manufacturing cost as well as the weight thereof while ensuring the rigidness of the main body.
An ordinary two-stroke internal combustion engine which is conventionally used in a portable power working machine such as a bush cutter or a chain saw is generally provided with a main body essentially consisted of a cylinder portion in which a piston is slidably inserted, and a crankcase portion in which a crank shaft is rotatably supported via a bearing. In this case, the cylinder portion is formed separate from the crankcase portion, and the crankcase portion is formed of a vertical two-part (vertical split type) structure or a horizontal two-part (horizontal split type) structure. Further, a fan case for forced cooling fan for engine is usually attached to the crankcase portion or to the cylinder portion.
As described above, according to the conventional two-stroke internal combustion engine, the main body thereof is constituted by three or four pieces, and the fan case is required to be subsequently attached to the crankcase portion or to the cylinder portion by making use of bolts, etc. As a result, a large number of assembling bolts are required for assembling the main body, and the working and assembling of the main body is troublesome, thus making it difficult to reduce the weight and manufacturing cost of the main body.
The present invention has been made to overcome the aforementioned problems accompanied with the conventional two-stroke internal combustion engine, and therefore, an object of the present invention is to provide a two-stroke internal combustion engine which is capable of reducing the number of pieces constituting the main body, thereby making it possible to reduce the number of parts such as assembling bolts, etc. and hence to reduce the manufacturing cost as well as the weight thereof while ensuring the rigidness of the main body.
With a view to realize the aforementioned objects, the two-stroke internal combustion engine according to the present invention fundamentally comprises a main body essentially consisting of a cylinder portion in which a piston is adapted to be fittingly inserted; a crankcase portion in which a crank shaft is rotatably supported via a bearing.
The crankcase portion is featured in that it is split into an upper crankcase portion and a lower crankcase portion by a horizontal plane which passes through the central axial line of the crankshaft, wherein the upper crankcase portion is integrated with the cylinder portion, thus forming an integral cylinder-case, and a fan case base is formed integral with the lower crankcase portion.
In a preferred embodiment of the present invention, the fan case base is integrally provided with a reinforcing bridge portion overhanging the crankshaft, both end portions of the reinforcing bridge portion being designed to be fastened by means of bolts, etc. to the cylinder-case.
On the occasion of assembling the two-stroke internal combustion engine of the present invention which is constructed as described above, the crankshaft and the bearing are interposed between the upper crankcase portion of the cylinder-case and the lower crankcase portion, and at the same time, one end portion of the crankshaft is disposed below the reinforcing bridge portion of the fan case base. Then, the cylinder-case is fastened to the lower crankcase portion by means of bolts, etc. and at the same time, the both end portions of the reinforcing bridge portion are fixed to the cylinder-case by means of bolts, etc.
According to the two-stroke internal combustion engine of the present invention as described above, the main body inclusive of the fan case base is constituted by only two pieces, and the fan case base which is integrally provided with the reinforcing bridge portion is designed to be fastened to the cylinder-case by means of bolts, etc. As a result, the number of parts such as assembling bolt, etc. can be reduced, and hence the working and assembling of the main body can be facilitated, thus making it possible to reduce the manufacturing cost as well as the weight thereof while ensuring the rigidness of the main body.